"I believe that the [study] results are credible and important," Gandy said. "Physical exercise is probably the best brain drug we have."

For the study, Segaert and her colleagues gave a computerized language test to 28 healthy British men and women whose average age was 67 to 70. The researchers also had 27 younger adults, average age 23, take the test.

The test asked for the names of famous people (for example, authors, politicians and actors) based on 20 questions about them. Study participants also were given definitions of 20 little-used words and 20 easy words, and asked to produce the corresponding word.

The researchers used a stationary bicycling test to gauge aerobic fitness. This assessed the ability to use oxygen during exercise.

"We found that the higher the older person's aerobic fitness level was, the lower their odds of experiencing a tip-of-the-tongue moment," Segaert said.

"Being fit thus seems to offer some protection against language decline," she said.

The older group had a significantly larger vocabulary than the younger adults. But even fitter seniors had more lapses in language than their juniors, the findings showed.

Language is a vital skill, Segaert said. And finding the sounds for words is essential when you want to produce language fluently.

"Speaking is something we rely on every day. Communication with others helps us maintain social relationships and independence into old age. Being fit may offer some help with that," she said.

Gandy said efforts are underway to understand the molecular neurobiology of exercise's effects on the brain.

Research has shown that physical exercise stimulates production of a brain-derived nerve growth factor capable of staving off brain aging in apes, he added.

"Brain-derived nerve growth factor is most active during development and after injury. But its general role is to nourish brain nerve cells and help them withstand injuries, including exposure to amyloid poisoning in Alzheimer's," Gandy said.

The nerve growth factor also protects nerve cells from age-related shrinkage and wasting, he added.

Some or all of these effects may be at play in these current findings, Gandy explained.

So how much exercise is best?

The researchers pointed out that you can measurably increase your fitness level in 6 weeks. And even walking has been shown to have "cognitive," or mental, benefits.

Generally, moderate exercise is advised for 30 minutes a day most days of the week.

The report was published online April 30 in the journal Scientific Reports.